The present invention relates to a structure of a protective circuit for use in a semiconductor device and, in particular, to an overcurrent protective circuit to protect a switching element in a semiconductor device using an insulated gate type semiconductor switching element such as an IGBT from an overcurrent.
FIG. 12 shows a circuit diagram of a conventional semiconductor device which includes an overcurrent protective circuit. The conventional semiconductor device includes an insulated gate type bipolar transistor (which will be abbreviated as IGBT) 10 as a switching element and a current limit circuit 20. The IGBT 10 is a power semiconductor device which is able to control a large current under a high voltage. In the semiconductor device, an external terminal P1, which provides a high potential, is connected to a collector 11 of the IGBT 10, and an external terminal P2, which provides a low potential, is connected to an emitter 12 of the IGBT 10. And, by controlling a gate potential Vg to be applied to a gate electrode 13 of the IGBT 10, the current of a load circuit to be connected to the external terminals P1 and P2 can be controlled. Further, the IGBT 10 includes a sensing emitter 14 which is used to sense or detect a current, in addition to the emitter 12 to be connected to the external terminal P2. That is, the IGBT 10, as shown in FIG. 13 in terms of an equivalent circuit, includes a main insulated gate type switching element (main IGBT) T.sub.1 and a current-detecting insulated gate type switching element (sub IGBT) T.sub.2 connected in parrell to the main IGBT. The emitter of the current-detecting insulated gate type switching element T.sub.2 is the sensing emitter 14. The gate electrode 13 comprises a gate electrode 13a of the main insulated gate type switching element T.sub.1 and a gate electrode 13b of the current-detecting insulated gate type switching element T.sub.2. The sensing emitter 14 is connected to the external terminal P2 through a current sensing resistor 21. For this reason, from the sensing emitter 14, a current is allowed to flow out which is proportional to a current flowing between the collector 11 and emitter 12 of the main insulated gate type switching element T.sub.1. The current limit circuit 20 comprises the current sensing resistor 21, a diode 35 for preventing an inverse current, and an n-channel MOSFET 30 connected through the inverse current preventive diode 35 to a gate line 15 to which a gate control signal is supplied by a gate drive circuit (not shown). The n-channel MOSFET 30 includes a source 31 which is connected to the external terminal P2 providing the low potential, a drain 32 which is connected through the inverse current preventive diode 35 to the gate line 15, and a gate 33 to which a drop voltage Vs in the current sensing resistor 21 is applied.
In the above-mentioned current limit circuit 20, if an overcurrent is caused to flow in the IGBT 10 due to shorts or the like and a given current flows from the sensing emitter 14 to the current sensing resistor 21, then the drop voltage in the current sensing resistor exceeds a threshold voltage of the MOSFET 30. As a result, the MOSFET 30 turns on, so that the current to be applied to the gate 13 of the IGBT 10 through the gate line 15 is caused to pass through the MOSFET 30, that is, the current is bypassed. Therefore, the gate potential Vg to be applied to the gate 13 decreases to thereby limit a collector current passing through the IGBT 10.
As mentioned above, the semiconductor device including the current limit circuit 20 is able to protect the main switching element from the overcurrent. However, in the conventional circuit, when executing current limit control in the main switching element, the current is lowered suddenly and sharply in order to protect the main switching element speedily.
FIG. 14 shows the collector current flowing through the external terminal P1 and the drop voltage Vs occurring in the current sensing resistor 21 when the short of the load circuit or the like causes a large current to flow in a device using the current limit circuit shown in FIG. 12. At first, if the large current flows in the IGBT 10, then a current proportional to the large current flows from the sensing emitter 14 and the drop voltage Vs in the current sensing resistor 21 also rises. And, at a time t.sub.10, if the drop voltage Vs exceeds the threshold voltage V.sub.th of the MOSFET 30, then the MOSFET 30 is allowed to conduct.
However, the collector current starts to decrease at a time t.sub.11 when an overcurrent flows slightly due to delay in response in the operation of the MOSFET 30. At that time, especially in an element which controls a large current, due to the time differentiation (di/dt) of the suddenly lowering current and a wiring inductance L on the circuit etc., an induced voltage of L.times.di/dt, which is an inductance load voltage, occurs in the current sensing resistor 21. As a result of this, at a time t.sub.12 as well, the MOSFET 30 is continuously biased forwardly by the induced voltage so that a voltage between the drain 32 and source 31 is lowering continuously. Therefore, the gate potential Vg applied to the IGBT 10 is further lowered to become under the threshold voltage of the IGBT 10 and thus, at a time t.sub.13, the IGBT 10 is turned off. If the IGBT 10 is turned off once in this manner, although an overcurrent does not flow, not only the drop voltage Vs in the current sensing resistance 21 lowers down to zero but also the MOSFET 30 is turned off. Therefore, the gate potential Vg returns to a given potential which is supplied by the gate drive circuit, so that the IGBT 10 is turned on again to provide a condition which allows an overcurrent to flow. As mentioned above, in the conventional current limit circuit, especially when treating a large current, the opening and closing of the main switching element is repeated while the current is limited, which may cause the current value to oscillate.